Gas-discharge tube with a pool cathode and fluted anode



C. R. WETTER May 26, 1964 GAS-DISCHARGE TUBE WITH A POOL CATHODE. AND FLUTE-D ANODE Filed Jan. 2, 1962 Fire 4 FIG. 5

INVENTOR. Charles waiter wwwww yj United States Patent 3,134,921 GAS-DISCHARGE TUBE WITH A POOL CATHODE AND FLUTED ANGDE Qharles R. Wetter, Geneva, Ill., assignor to National Electronics, Inc, Geneva, lli., a corporation of Illinois Filed Jan. 2, 1962, Ser. No. 163,769 9 Claims. (Cl. 313l71) This invention relates to gas-discharge tubes of ignitron type; in particular, it concerns a tube of that class in which the conformation and orientation of the parts cooperate to provide greatly improved performance at high peak currents.

Ignitron tubes per se are of course well known and widely used. They comprise (1) an evacuated envelope, usually made primarily of metal, (2) a mercury-pool oath ode, (3) an anode disposed within the envelope above the mercury pool, (4) suitable terminal connections permitting an external electrical circuit to be connected to the anode and cathode respectively, and (5) an ignitor electrode disposed above and in contact with the mercury pool, together with a suitable external connection terminal therefor.

Conventional ignitrons also, as a rule, have a means for liquid cooling of the envelope during operation. Because the cooling means is not involved in the present invention, however, no further reference to such means need be made in this specification, it being understood that a suitable cooling arrangement will be provided in connection with all of the embodiments of the invention herein described.

While the anode and the ignitor electrodes are insulated from the envelope of an ignitron, the mercury-pool cathode is at the same potential as, and electrically connected to, the tube envelope.

The conduction of current through an ignitron under various conditions of operation is a highly complex phenomenon which may involve two or more modes of conduction operative simultaneously. The mode of conductin normally thought of as predominant takes place through an electric are extending between the anode and the cathode arc spots on the surface of the mercury pool. Such an arc can form when (a) the potential of the anode is positive relative to the cathode and (b) ionization of mercury vapor within the tube envelope has been initiated by application of a suitable current pulse through the circuit defined by the ignitor electrode and the cathode.

Once arc conduction between the anode and cathode has been established in this manner, it will continue so long as the anode potential remains sufficiently positive relative to the cathode. If arc conduction ceases, however, the mercury vapor within the envelope will quickly recondense, and no new are will start, regardless of anode potential, until and unless a new trigger pulse is applied to the ignitor.

The general principles of ignitron operation, as above described, are well known in the art.

The current-carrying capabilities of any given ignitron tube are normally expressed in terms of ratings as to average current and peak current. The principal factor determining the average current which an ignitron can carry is the ability of the structure to dissipate the heat generated therein. The limitation on peak current, however, is primarily determined by the tubes ability to maintain the cathode arc spots on mercury pool as the source of emission. When the peak current increases beyond a critical limit, the arc will shift, in whole or in part, from the mercury pool to the metal walls of the tube envelope. This formation of a cathode are spot on the envelope wall is known in the art as are transfer.

The reason ignitron tubes, particularly those embody- 3,134,921 Patented May 26, 1964 ing prior-art designs, should be operated at peak currents below the level at which are transfer will take place is that are conduction from the envelope wall to the anode results in evaporation of metal ions from the wall into the gas plasma within the tube. When conduction ceases, the mercury vapor within the tube promptly recondenses into liquid mercury and rejoins the cathode pool, but the evaporated metal atoms do not, in general, return to the envelope walls. They are deposited on surfaces throughout the tube interior, and some of them find their way to the surface of the ignitor electrode. In consequence, when a tube is operated at peak currents above the arc-transfer level, the ignitor gradually becomes coated with a thin layer of metal evaporated from the envelope walls, and the resistance between the ignitor of the mercury pool is thereby progressively reduced. This, in turn, requires that more and more energy be employed to initiate are conduction, and ultimately renders the tube useless when such energy demands for ignition become excessive.

This condition, which results in greatly shortened tube life, occurs only when the tube is operated under aretransfer conditions, and the speed with which tube deterioration takes place is dependent on the quantity of arc current that is transferred by are conduction from the tube walls, as opposed to the mercury pool.

A major object of the present invention is to provide an ignitron design in which the conformation of the anode and the relative orientation of the anode and other tube components cooperate to reduce greatly the tendency of the arc to transfer from the mercury pool to the walls. Stated diiferently, the ignitron design of the present invention results in a substantial increase in the permissible peak current that may be carried by a given tube before are transfer will take place.

As a further object of the invention, my unique ignitron design insures that a substantial part of the total anode current will be conducted between the mercury pool and the anode even at peak currents whereat arc transfer occurs. This characteristic of my improved ignitron tubes results in great prolongation of service life even when the tubes are operated at currents at which are transfer occurs.

Still another object and advantage of my invention is that ignitrons embodying my design are characterized, during arc conduction, by a relatively dense mercuryvapor plasma throughout the greater part of the tube vol ume. This permits a substantial part of the load current to be carried between the envelope walls and the anode in the form of ion-neutralization current, as opposed to are conduction. Such current will flow whenever electrodes of differing potentials are immersed in a plasma of ionized gas, since under such circumstances positively charged gas ions will be drawn by electrostatic attraction toward the more negative electrode and correspondingly free electrons and negatively charged ions will migrate under electrostatic forces toward the positively charged electrode.

While the existence of ion-neutralization current in ignitrons has been known in the prior art, its potential usefulness has never, I believe, been fully appreciated. I have found that the ion-neutralization current may amount to as much as 10 amperes per square cm. of effective wall area. In prior-art ignitron designs, the wall area available for effective ion-neutralization conduction has been relatively small, because the mercury-vapor plasma has been confined largely to the space between the mercury pool and the lower end of the anode. In the ignitrons of the present invention, however, the plasma density in the portion of the tube above the lower edge of the anode is far greater than in prior-art designs, with the result that the ion-neutralization conduction is far more significant than in prior-art tubes.

The above-mentioned important advantages of my new ignitron design stem in part from the fact that in my tubes the arc drop-i.e., the voltage drop between anode and envelope-is as great or greater after arc transfer has occurred than is the normal arc drop between anode and mercury pool for comparable load currents. This desirable condition prevents are transfer from occuring except at extremely high peak currents and insures, even after arc transfer, that the bulk of the load current is still, nonetheless, carried in the form of anode-pool are conduction and ion-neutralization conduction. Since, even under arc-transfer conditions, only a small part of the total load current is carried by wall-to-anode are conduction, the rate at which ignitor contamination takes place in my tubes is but a small fraction of that encountered under like conditions with priorart tubes.

The improved performance of ignitrons embodying my invention is the cooperative result of the use therein of a fluted anode and the employment of a hithereto unconventional orientation of the anode with respect to the envelope walls and the mercury pool.

By employing an anode having a plurality of axial flutes, as opposed to the cylindrical design commonly used, I achieve several results that contribute significantly to the improved performance characteristic of my invention.

For one thing, the axial spaces between the flutes provide effective chimneys to carry upward in the tube the mercury-vapor plasma that boils otf the mercury pool during period in which the tube is conducting. This results in the presence of a relatively dense plasma throughout most of the space that laterally separates the anode from the tube envelope, thus providing a vastly improved environment in which ion-neutralization conduction may take place. This desirable condition is further enhanced by the fact that the surface area of the anode itself is much greater, with my fluted construction, than with the commonly used cylindrical design.

A second advantage achieved by my use of a fluted anode is that the average lateral distance separating the anode and the envelope walls is substantially greater, tube size for tube size, than in prior-art designs where a cylindrical anode is used. This results in an increased arc drop from wall to anode when the tube is operating under arc-transfer conditions.

The third advantage that results from my fluted anode construction is that it provides convenient clearance between the flutes for the ignitor electrode and its associatcd supports, thereby making it possible to dispose the anode in a position whereat its axial separation from the mercury pool is less than the over-all height of the ignitor assembly. This spatial disposition of the anode relative to the mercury pool reduces the characteristic are drop from anode to pool for any given load current-one of esirable objectives achieved by my invention.

A further advantage of the fluted anode construction is that its greater length facilitates the transfer of heat from the anode to the tube Walls by radiation. I am aware, however, that fluted anodes per se have been pre viously used for this purpose in electron tubes of various types, and hence this last-mentioned advantage is mentioned merely as an incidental benefit rather than as one of the objects of the invention.

In the appended drawing I have shown two representative embodiments of ignitron tubes according to my invention, the showing in both cases being semi-diagrammatic in that immaterial details such as cooling jackets, terminal connections, and the like have been omitted. In FIG. 1 I show, partly in section, a form of my invention in which the anode is provided with eight flutes, alternate ones of which are terminated at a position above the lowermost extremity of the anode. FIG. 2 is a sectional view of the FIG. 1 tube, the section being taken along the line 22 of FIG. 1. FIGS. 3 and 4 are respectively sectional views of the anode of the FIG. 1 device, the sections being respectively along the lines 33 and 4-4 of FIG. 2. FIG.

4 5 shows, partly in section, an alternative ignitron construction in which an anode having seven flutes is employed, all the flutes or fins in this case extending downward to the lowermost extremity of the anode. FIG. 6 is a sectional view of the FIG. 5 tube, the section being taken along the line 66 of FIG. 5.

Referring now to FIG. 1, I show therein an ignitron tube comprising a conventional cylindrical evacuated envelope 10 made of iron or stainless steel, provided with a bottom plate 11 peripherally welded to the wall of envelope It). A top plate 12 is similarly peripherally welded to the wall of envelope 10, the top plate 12 being centrally relieved to receive the anode supporting structure which comprises an annular glass insulator 13 sealed by a suitable glass-to-metal seal to a pair of metal members 14 and 15. The element 15, generally cylindrical in shape, is welded at its lower extremity to the top plate 12, while the element 14 is centrally relieved to receive a support rod 16, normally made of metal, from the lower end of which the anode 20 is supported centrally of the envelope 10. A conventional seal is provided between the element 14 and the rod 16 to provide mechanical support for the rod 16 while maintaining the gas-tight integrity of the space inside the tube.

It will be understood, as previously mentioned, that any suitable cooling jacket may be provided for the envelope 10; similarly, conventional contact electrodes may be provided to facilitate the connection of external circuit elements to the rod 16 (anode connection) and the bottom plate 11 (cathode connection).

The bottom plate 11 is relieved in one relatively small zone to receive the ignitor assembly, which comprises a glass support 21 that is ring-sealed to a metal element 22, which in turn is peripherally welded to the surface of bottom plate 11. Passing through the glass support member 21 is a conductor 23, the bottom end of which is modified to define a connection terminal 24 and the upper end of which, extending into the interior of the tube, carries a laterally extending arm 25 from which the ignitor element 26 depends. The upper surface of bottom plate 11 is covered by a pool of liquid mercury 27, extending upward in the envelope 10 to a point just above the lower end of ignitor 26.

The anode 20 may be made of graphite or other suitable material. Like conventional ignitron anodes, it carries an axially disposed threaded aperture at its upper end to receive the lower end of rod 16, which is provided with mating threads. The anode 20 comprises a central cylindrical core, designated on FIGS. 3 and 4 as 28, which extends the full length of the anode, terminating at a point substantially below the upper extremity of the ignitor assembly. Flanking the central core 28 and integrally formed therewith are eight flutes or fins. Four of these fins, designated 31 on the drawing, extend the full length of the core 28, while the other four flutes, designated 32, terminate short of the lower extremity of core 28 and substantially above the uppermost extension of the ignitor assembly.

As may be best seen from FIGS. 2-4, the flutes 31 and 32 extend radially beyond the core 28 for a substantial distance, adding greatly to both the surface area and the mass of the anode.

The anode 20, as previously mentioned, is axially disposed within the envelope 10 in such a position relative .to the mercury pool 27 that the spacing between the the arc drop from pool to anode when the instantaneous load current equals or exceeds the arc-transfer level. This condition is met by the particular relative orientation shown in the drawings; it is to be understood, however, that the illustrated conformation is intended to be merely illustrative rather than limiting.

FIGS. 5 and 6 illustrate another embodiment of my invention, generally like the FIG. 1 embodiment save for the shape of the anode. In the FIG. 5 form of the invention, the anode 40 is of cylindrical conformation in its uppermost part but is axially recessed throughout its lower portion to define seven fins or flutes 41, disposed around the central core 48 at 45 angle increments, save for one pair of flutes, designated 41a and 41b, which re spaced apart by 90. The enlarged space provided by this last-mentioned construction provides clearance for the ignitor assembly 45, which is mounted in the bottom plate 11 of the tube in the same general fashion as in the FIG. 1 embodiment.

As with the FIG. 1 tube, the anode 40 of the FIG. 5 embodiment is axially disposed within the envelope in such a position that its lower extremity extends below the upper end of the ignitor assembly, at a position whereat the minimum spacing between the lower end of the anode 40 and the pool 27 is less than the lateral spacing between the envelope 10 and the core portion 48 of the anode 40.

The number of flutes employed in the anodes of tubes embodying my invention may be varied considerably without sacrificing the advantages of the invention. I have found experimentally, however, that seven or eight flutes provide the optimum tube characteristics from the points of view of good heat radiation from the anode, resulting in a desirably high average-current rating, and large wall-to-anode arc drop, yielding a desirably high peak-current rating. If the number of flutes is reduced much below seven, it becomes diflicult to design a tube having satisfactory average-current rating, while an increase in the number of flutes much beyond eight reduces to an undesirable extent the waJl-to-anode arc drop under arc-transfer conditions.

When an ignitron tube embodying my invention is operated at relatively low peak currents, its behavior is generally similar to that of a conventional ignitron tube. As the peak current is increased, however, the superiority of my invention over conventional tubes shows itself strikingly.

For one thing, my deign, as above described, yields a tube in which the critical arc-transfer current is higher by a substantial factor than in conventional tubes of like size. Thus, when my invention is employed, the ratio of rated peak current to rated average current can be considerably greater than in conventional tubes.

Secondly, in tubes of my invention, only a small part-- perhaps 20% or lessof the load current is carried by wall-to-anode arc conduction even when the peak current is above the critical arc-transfer level. This desirable condition results in much slower ignitor contamination and hence much longer tube life, even in applications where the tube is operated at peak currents above the rated value. This last-mentioned characteristic of tubes embodying my invention is in sharp contrast to the behavior of ordinary ignitron tubes operated at peak currents above the transfer level, for in those tubes, once arc transfer has occurred, the great bulk of the load current is carried by wall-to-anode arc conduction.

The improved characteristics of my tubes when operated under arc-transfer conditions, are due, I believe, to two principal factors. First, the wall-to-anode voltage drop in my tubes is always equal to or greater than the pool-to-anode voltage drop, with the result that the pool continues to supply its full share of load current even after arc transfer. Second, the chimney effect provided by my fluted anode construction yields a relatively dense mercury-vapor plasma throughout virtually all the space laterally separating the anode and the envelope walls, thus enabling a substantial part of the load current to flow between the electrodes in the form of ion-neutralization current rather than by arc conduction.

While my experimental data indicate that the foregoing are the primary explanations for the improved perform ance of ignitron tubes embodying my invention, I do not wish to be bound by that explanation or limited thereto, since there may be other causative factors involved. Be that as it may, my experience has demonstrated beyond doubt that tubes embodying the principles of my invention do in fact perform far better and last much longer than conventional tubes when operated under arc-transfer conditions.

Skilled readers, on learning the design principles herein taught, will be able to depart considerably from the precise structures shown in the drawings as preferred embodiments of my invention, while retaining many or all of the advantages thereof. With that in mind, therefore, it is to be understood that the terms flute and fluted as herein used are to be taken in a broad sense which embraces all anode configurations characterized by alternate zones of relatively greater and lesser radial separation from the envelope walls, effective to provide the plasma chimneys and increased wall-to-anode arc drop which are distinctive features of my invention. With respect to other features of my invention as Well, the illustrated embodiments of my invention are intended to be merely illustrative.

I claim:

1. In a gas-discharge tube of the ignitron type having an evacuated generally cylindrical conductive envelope, a mercury pool at the bottom thereof, an ignitor assembly extending above the mercury pool, and means for insulatedly supporting an anode within the envelope above the mercury pool, the improvement which comprises an anode having a central core and a plurality of flutes in angularly spaced relation extending radially from said core toward the inner surface of said envelope, said anode being secured to said supporting means in a position whereat the maximum lateral separation between the inner wall of said envelope and the surface of said anode is at least as great as the minimum separation between the lower end of said anode and the surface of said pool.

2. The apparatus of claim 1 wherein the number of said flutes is between six and nine, inclusive.

3. In a gas-discharge tube of the ignitron type having an evacuated generally cylindrical conductive envelope, a mercury pool at the bottom thereof, an ignitor assembly extending above the mercury pool, and means for insulatedly supporting an anode within the envelope above the mercury pool, the improvement which comprises an anode having a central core and a plurality of flutes in angularly spaced relation extending radially from said core toward the inner surface of said envelope, said anode being secured to said supporting means in a position whereat the maximum lateral separation between the inner wall of said envelope and the surface of said anode is greater than the minimum separation between the lower end of said anode and the surface of said pool.

4. The apparatus of claim 3 wherein the minimum separation between the lower end of said anode and the surface of said pool is less than the maximum upward extension of said ignitor assembly above said pool, said flutes being positioned to provide clearance for said ignitor assembly.

5. In a gas-discharge tube of the ignitron type having an evacuated generally cylindrical conductive envelope, a mercury pool at the bottom thereof, an ignitor assembly extending above the mercury pool, and means for insulatedly supporting an anode within the envelope above the mercury pool, the improvement which comprises an anode having a central core and a plurality of flutes in angularly spaced relation extending radially from said core toward the inner surface of said envelope, said anode being secured to said supporting means in a position whereat the minimum separation between the lower end of said anode and the surface of said pool and the maximum lateral separation between the inner wall of said envelope and the surface of said anode are proportioned to provide a wall-to-anode arc voltage drop, for ignitron load currents exceeding the arc-transfer level, that at least equals in magnitude the pool-to-anode arc voltage drop at the same load current.

6. The apparatus defined in claim 5 wherein said minimum separation between said anode and the surface of said pool is less than the maximum upward extension of said ignitor assembly above said pool surface, said fiutes being positioned to provide clearance for said ignitor assembly.

7. The apparatus of claim 5 wherein the number of said flutes is between six and nine, inclusive.

8. In a gas-discharge tube of the ignitron type having an evacuated generally cylindrical conductive envelope, a cathode at the bottom thereof, an ignitor assembly extending above the cathode, and means for insulatedly supporting an anode within the envelope above the cathode, the improvement which comprises an anode having a central core and a plurality of flutes in angularly spaced relation extending radially from said core toward the inner surface of said envelope, said anode being secured to said supporting means in a position whereat the maximum lateral separation between the inner wall of said envelope and the surface of said anode is at least as great as the minimum separationbetween the lower end of said anode and the surface of said cathode.

9. In a gas-discharge tube of the ignitron type having an evacuated generally cylindrical conductive envelope, a cathode at the bottom thereof, an ignitor assembly extending above the cathode, and means for insulatedly supporting an anode within the envelope above the cathode, the improvement which comprises an anode having a central core and a plurality of flutes in angularly spaced relation extending radially from said core toward the inner surface of said envelope, said anode being secured to said supporting means in a position whereat the maximum lateral separation between the inner wall of said envelope and the surface of said anode is greater than the minimum separation between the lower end of said anode and the surface of said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 1,234,875 Conrad July 31, 1917 1,683,156 Brown Sept. 4, 1928 2,727,168 Marshall et al Dec. 13, 1955 

1. IN A GAS-DISCHARGE TUBE OF THE IGNITRON TYPE HAVING AN EVACUATED GENERALLY CYLINDRICAL CONDUCTIVE ENVELOPE, A MERCURY POOL AT THE BOTTOM THEREOF, AN IGNITOR ASSEMBLY EXTENDING ABOVE THE MERCURY POOL, AND MEANS FOR INSULATEDLY SUPPORTING AN ANODE WITHIN THE ENVELOPE ABOVE THE MERCURY POOL, THE IMPROVEMENT WHICH COMPRISES AN ANODE HAVING A CENTRAL CORE AND A PLURALITY OF FLUTES IN ANGULARLY SPACED RELATION EXTENDING RADIALLY FROM SAID CORE TOWARD THE INNER SURFACE OF SAID ENVELOPE, SAID ANODE BEING SECURED TO SAID SUPPORTING MEANS IN A POSITION WHEREAT THE MAXIMUM LATERAL SEPARATION BETWEEN THE INNER WALL OF SAID ENVELOPE AND THE SURFACE OF SAID ANODE IS AT LEAST AS GREAT AS THE MINUMUM SEPARATION BETWEEN THE LOWER END OF SAID ANODE AND THE SURFACE OF SAID POOL. 